Set your slow cooker on the low setting, and spray the inside with cooking spray.
Parboil the macaroni in salted water for 6 minutes. Meanwhile, shred cheese and toss together to combine.
While the macaroni is cooking, in a small mixing bowl, whisk together, the melted butter, evaporated milk, cheddar cheese soup, sour cream, eggs, ground mustard, black pepper and salt. Whisk together until fully combined.
After cooking the macaroni for 6 minutes, drain well. Add to the slow cooker.
Pour the cheese sauce over the macaroni along with 3 cups of shredded triple cheddar cheese blend. (Reserve final 1/2 cup to sprinkle on top.)
Stir all of the ingredients together until the macaroni is coated and the sauce is evenly distributed.
Cover and cook on low for 2 hours. After 2 hours, remove the lid and check the middle for doneness. If needed, add additional cooking time in 20-30 minute increments or until the macaroni is tender. (Time may vary slightly depending on how your slow cooker cooks)
Sprinkle the remaining cheese on top. Replace the lid and continue to cook for an additional 15 minutes to allow the cheese to melt.
Keep warm until serving.
To Make in the Oven: Boil the macaroni in salted water per the package directions until al dente. Drain well.Whisk together the sauce ingredients and prep the cheese.In a large bowl, combine the macaron, cheese sauce and 3 cups shredded cheese. Pour into a buttered 13x9-inch baking dish and top with the reserved cheese. Dot the top with pats of butter.Bake in a preheated 350°F for 40 minutes or until bubbly and golden brown.